Limestone is a low cost naturally occurring chemical which is the preferred reagent in many conventional WFGD systems. However, under normal system operating conditions, the limestone dissolves slowly. Therefore, in order to increase the alkalinity of the limestone slurry used in conventional WFGD systems, and hence increase the system's SO.sub.2 removal efficiency, excess limestone (typically ground to a particulate with 90-95% of the particles having a size of less than 325 mesh) is used and the reaction tank is oversized to allow sufficient retention time for limestone dissolution. Once the slurry containing the dissolved limestone is sprayed into the SO.sub.2 absorber, the dissolved limestone is quickly depleted making the slurry ineffective in removing more SO.sub.2 and requiring a high rate of slurry recycle. As a result, high capital and operating costs are required when low cost limestone is employed as the reagent in WFGD systems.
In order to improve WFGD system performance, other additives such as adipic acid and dibasic organic acids have been utilized. However, these additives are relatively expensive, tend to oxidize and cannot be recovered from waste water. While such additives improve system performance by buffering the slurry pH and improving its effectiveness, the performance enhancement that is achieved in many cases is not of a sufficient magnitude to justify the increased make-up cost of the additives.
Therefore, it would especially be desirable if the capital and operating costs of limestone-based WFGD systems could be reduced by the use of a low cost recoverable additive while, at the same time, achieving significantly increased limestone dissolution rates (and hence increased SO.sub.2 removal efficiency). It is towards providing such performance-enhanced limestone-based WFGD systems that the present invention is directed.
Broadly, the present invention is directed toward incorporating a source of ammonium ions in a limestone-based WFGD system. In this regard, it has been found that the presence of ammonium ions in an aqueous limestone-containing slurry increases the dissolution rate of the limestone and thereby enhances SO.sub.2 removal efficiency of the WFGD system in which such slurry is used.
Further aspects and advantages of this invention will become more clear after careful consideration is given to the following detailed description of the preferred exemplary embodiments thereof.